Previously only the _contents_ of modules were minified, but not the
module infrastructure that Browserify adds around it. Given lots of modules,
this adds up.

Replace `uglifify` with a small wrapper around terser, the package that
actually compresses the code, and use it to minify both the contents of
modules and the generated wrapper code. Calling terser directly allows
us to a) use the current version of it (uglifyify was stuck on Terser
3.x) and b) upgrade/configure/replace it more directly in future.

One other change here is that minifying modules is now done _after_ the
transform that replaces `if (process.env.NODE_ENV == ...)` checks with
`if (true)` or `if (false)`. This enables dead-code removal to remove
the logic entirely.

This reduces the minified size of `sidebar.bundle.js` from 368 KB to 311
KB (-15%) and `annotator.bundle.js` from 170 KB to 152 KB (-12.7%).
Some vendor bundles are smaller, others are unaffected because they were
already fully minified. The total minified size of all bundles is reduced by ~7%.